In cellular communications systems, the mobile radio station communicates over an assigned radio channel with a radio base station. Several base stations are coupled to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communication systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more switching nodes to a base station which serves the called mobile station. The base station pages the called mobile station and establishes a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction.
Due to the rapid expansion of wireless mobile communications and the need for wideband multimedia services, there is a continuing need to better utilize the available frequency bandwidth. A common strategy in Frequency Division Multiple Access (FDMA)/Time Division Multiple Access (TDMA) systems is to reuse the frequencies in the network. The challenge with frequency reuse is to counteract or at least reduce the interference between transmitters in the system using the same frequency by controlling the transmit power levels of the radio signals and by separating to the extent practical the transmitters by a sufficient geographic distance. The transmit power levels of the mobile stations and base stations are ideally lowered so that only the minimum transmission power necessary to maintain satisfactory call quality is used. By reducing mobile and base station transmission power, the other radio communicators experience lower interference which means that the system capacity may be increased.
In a Code Division Multiple Access (CDMA) mobile communication system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same radio frequency band—hence the term “spread spectrum.” In other words, individual radio “channels” are discriminated upon the basis of these codes. Because all users of a CDMA communications system transmit information using the same frequency band at the same time, each user's communication interferes with the communications of the other users. In addition, signals received by a base station from a mobile station close to the base station are much stronger than signals received from other mobile stations located at the base station's cell boundary. As a result, distant mobile communications may be overshadowed and dominated by close-in mobile stations.
Interference is a particularly severe problem in CDMA systems. If one mobile station transmits at a power output that is too large, the interference it creates degrades the signal-to-interference ratio (SIR) of signals received from other mobile radios to the point that a receiving base station cannot correctly demodulate transmissions from the other mobile radios. In fact, if a mobile station transmits a signal at twice the power level needed for the signal to be accurately received at the base station receiver, that mobile's signal occupies roughly twice the system capacity as it would if the signal were transmitted at the optimum power level. Unregulated, it is not uncommon for a “strong” mobile station to transmit signals that are received at the base station at many, many times the strength of other mobile transmissions. Such a loss of system capacity to excessively “strong” mobile stations is unacceptable.
Additional problems are associated with excessive transmit power. One is the so-called “party effect.” If a mobile transmits at too high of a power level, the other mobiles may increase their respective power levels so that they can “be heard,” compounding the already serious interference problem. Another problem is wasted battery power. It is very important to conserve the limited battery life in mobile radios. The major drain on a mobile's battery occurs during transmission. A significant objective for any power control approach, therefore, is to reduce transmit power where possible.
Most radio transmit power control procedures try to keep the signal strength and/or quality of the signal detected by a receiver above a desired threshold without using unnecessarily high transmit power. Because power control is so important, CDMA systems employ a relatively high sampling rate for the power control algorithm, e.g., 1500 times per second. To minimize overhead control signaling, only one bit is used to communicate power control adjustments to the radio transmitter. The power is stepwise increased or decreased based upon a comparison of received signal strength, or some other signal parameter, with a threshold. The receiver controls the transmitter's power by issuing transmit power control commands (TPCCs)—power up or power down—at the same high sampling rate (e.g., once every 0.667 msec) based on signal quality measurements, e.g., signal-to-interference ratio (SIR). If the measured signal parameter value is less than a target signal parameter value, the power-up command is issued; otherwise, the power-down command is issued. The radio transmitter responds to the power control commands by increasing or decreasing its transmit output power level P, for example, by a certain incremental power step Δ, i.e., P←P+Δ or P←P−Δ.
Because a power-up or a power-down command issues every 0.667 msec, the transmit power level is never constant or static. Accordingly, even in an ideal radio environment, the incremental power control commands continually alternate between power up and power down so that the transmit power level and the received signal quality oscillate up and down an incremental step around a target value. In order to maintain the quality of the received signal always above a prescribed limit, the target value needs to be set slightly higher than that limit so that the received signal quality after the power down step is still above the prescribed limit.
Uplink capacity is limited by a maximum, acceptable level of uplink interference. Uplink interference depends on the current uplink traffic load, movement of the mobile station (mobility), and current radio conditions. Traffic load can vary considerably and rapidly, especially with multimedia and other “bursty” data services. Mobility and changing radiowave propagation conditions affect the quality of communications channels, and thus, the proper power level needed to transmit over those channels.
FIG. 1 shows a mobile communications environment with multiple, direct and indirect radio signals between base stations and mobile stations. FIG. 2 illustrates that the overall signal strength of a received signal varies as the distance between the transmitter and the receiver and depends (in one well-established model) on three variable factors: path loss, shadowing, and multipath fading. Path loss is the overall decrease in the field strength of the transmitted signal as the distance between the transmitter and the receiver increases. Shadowing occurs as a result of obstructions between the transmitter and receiver such as buildings, trees, and other objects in the environment. Multipath fading occurs as a result of constructive and destructive interference between multiple waves (rays) reaching the receiver from the transmitter. Multipath fading is particularly troubling because the quality of the received signal varies so rapidly.
In light of these obstacles, there is a need to dynamically control uplink interference so that satisfactory service is provided and maximum capacity is achieved. One way to control uplink interference is to assign maximum bit rates to mobile transmitters based on the current uplink interference load. But maximum bit rates do not adapt to changing conditions. Moreover, when most or all mobiles are transmitting at their maximum bit rate, there will be a spike in the uplink load perhaps causing less than satisfactory service. At other times, many of the mobiles may not transmitting at all, or at less than maximum bit rate, causing a dip in the uplink load and unused capacity.
Another approach is to limit the amount of time that mobiles can transmit in the uplink direction. A mobile station transmitting only 50% of the time generates less interference than when transmitting 100% of the time. One way to implement time-limited transmission is to assign each mobile station a probability of transmission and have each mobile transmit in a random fashion to meet this probability. EP 1033846 A1 describes broadcasting access probabilities to mobiles. Each mobile compares a random number to the probability. Uplink transmission occurs only when the random number is less than the broadcast probability.
Although this approach statistically reduces the uplink load, its randomness means that there is no attempt to transmit at particular times and not at others. As a result, the random time chosen to transmit may very well correspond to a time when radio transmission conditions are unfavorable, e.g., during a fading dip or when the current uplink load is momentarily high. An unfavorable condition means that the mobile must increase its transmit power, if possible, causing increased battery drain and interference or decreased signal quality at the base station.
These problems are overcome by limiting the amount of time that a mobile station can transmit and timing transmissions so they occur during favorable channel conditions rather than during less favorable channel conditions. Transmitting during favorable channel conditions requires less power, which translates into lower battery consumption and less interference to other uplink transmissions. Transmitting during favorable channel conditions also means fewer bit errors in received transmissions. Less uplink interference translates into a reduced uplink traffic load and more uplink capacity. If desired, the additional capacity may be used to allow mobile terminals to transmit at higher bit rates than would otherwise be permitted/possible in unfavorable channel conditions.
Having the mobile terminals transmitting during favorable channel conditions is particularly beneficial because fast channel variations due to constructive and destructive multipath fading are typically uncorrelated between mobile stations. If mobile stations only transmit during constructive fades (rather than during destructive fades), a higher instantaneous bit rate can be used to maintain the same average bit rate as when transmitting continuously. Because fading is uncorrelated between the transmitting mobiles, the number of simultaneously transmitting mobile stations will, on average, decrease. In this way, interference is reduced.
The radio network provides centralized load control in order to control and limit uplink interference in a cell while at the same time permitting adaptation to fast channel variations. The radio network sends to a mobile terminal a percentage of time that that mobile terminal may transmit over a radio channel in the uplink direction to the radio network. The mobile determines a current or future condition of the radio channel and restricts uplink transmissions based on the received percentage and the radio channel condition. Transmissions are restricted to times when the current or future condition is favorable, up to the percentage amount. Favorability may be determined by the mobile station, in one example embodiment, by comparing the radio channel condition to an average condition of the channel, such as a local or moving average channel condition.
Based on the load for the overall cell, an activity factor corresponding to a fraction or percentage is determined for each of the mobile terminals. The activity factor defines an amount of time relative to a total time that the mobile terminal may transmit based upon desired uplink load level for the cell area. The activity factors need only be sent to the mobile terminals infrequently thereby keeping the signaling load low. Activity factors could be sent more frequently, if desired, depending upon changing load conditions. Moreover, one activity factor may be assigned to a group of mobiles or to all mobiles in a specific cell, group of cells, or even in an entire radio network. Broadcasting group-specific, cell-specific, and network-specific activity factors reduces signaling. In any event, the mobile terminals are permitted to implement an appropriate algorithm to determine the best times to transmit when favorable channel conditions are present for that particular mobile terminal.
Information regarding the current or future condition of the radio channel is provided, in an example, non-limiting embodiment, from the radio network, preferably at a high frequency. Because transmit power control commands (TPCCs) are sent from the radio network very frequently already, TPCCs are advantageously used by the mobile terminals, in a preferred example embodiment, to determine when favorable channel conditions exist or will exist. One of the benefits of using TPCCs is that the radio network can control the uplink load without having to send additional control signaling messages to each of the mobile terminals to indicate when each mobile terminal should transmit and when it should stop transmitting.
One example algorithm that uses TPCCs to determine channel conditions calculates a cumulative transmit power control command (CPTCC) value. The network provides an activity factor used to determine a transmit threshold. The cumulative TPCC value is compared to the transmit threshold, and the determination of when to transmit is based on the comparison. When the percentage of allowed transmit time is higher, the transmit threshold is higher, and when the percentage is lower, the transmit threshold is lower.
Because transmissions occur during favorable channel conditions when there is less interference and because those favorable channel conditions are not correlated between different channels, the overall uplink cell interference level decreases. This permits the mobiles to increase their bit rate up to a preset maximum, if desired, or conserve battery power. If the interference level is reduced, the cell capacity increases. These advantages are not achieved when transmissions are timed randomly.